1. Field of the Invention
This invention relates in general to the field of dispensing apparatus which dispenses liquids, including liquid adhesives and in particular to a method and apparatus for accurately controlling the amount of liquid dispensed relative to the amount of liquid required to be dispensed.
2. Description of the Prior Art
Dispensing apparatus, and in particular, adhesive dispensing apparatus has in the past been used to dispense a liquid adhesive for applying a discrete quantity of adhesive at predetermined time intervals to such products as paper cartons, baby or adult diapers, rolls of paper towels, rolls of bathroom tissue and other like products as the products move along a product conveyor line. The liquid adhesive is dispensed during the various stages of manufacturing for such purposes as securing the towels or bathroom tissue to the paper rolls, for securing the final wrap of towels or bathroom tissue to the full roll, for fastening together the multiple layers of disposable diapers, etc. Adhesive dispensing apparatus typically includes an adhesive storage or supply tank, one or more adhesive applicator heads, flow lines for connecting the adhesive supply to the adhesive applicator head, a pump for supplying the driving force to cause the flow of the adhesive, and apparatus for controlling the operation of the applicator head to achieve a desired pattern of application of the adhesive.
In the event a hot-melt adhesive is used, the apparatus further includes heating, insulation and additional control means to maintain the adhesive in a liquid state within each of the components comprising the adhesive dispensing apparatus. Hot-melt adhesives generally produce stronger bonds and cure more rapidly than comparable cold adhesives. In commercial production line applications where both high volume and high quality are necessary, hot-melt adhesives are therefore preferred. As previously noted, however, hot-melt adhesives require additional apparatus and controls to assure proper application of the adhesive to a product to achieve the desired result.
In the above-described applications, the dispensed adhesive may be in the form of small dots, thin dashed or solid lines, large dots, broad dashed or solid lines, etc. The lines or dots of adhesive are usually applied in a direction coincident with the direction of movement of the product as it travels past a fixed position of the adhesive applicator. In applications where a series of multiple lines or dots of adhesive are to be applied, a number of applicators may be ganged together or a multiple outlet adhesive applicator may be used. The adhesive applicator is usually the only component of the dispensing system which is located in the immediate vicinity of the product. The remaining components, for example the supply tank, pump, etc., are usually remotely located relative to the product.
Typically, in the prior art, control apparatus is used to insure the maintenance of the correct temperature of the adhesive and to insure that the dispensing apparatus dispenses the correct amount of adhesive at the correct location on the product. One such example of the prior art control apparatus is shown in U.S. Pat. No. 3,408,008, issued Oct. 29, 1968 by Eric H. Cocks, entitled "Apparatus for Applying Hot-Melt Adhesives."
In other prior art methods and apparatus for controlling the dispensing of the adhesive a shaft speed encoder or other similar transducer is used to time the dispensing of the adhesive relative to the speed of the product passing by the adhesive applicator. The shaft speed encoder is typically attached to the shaft of a motor or to a shaft forming part of the product conveyor line. The output of the shaft speed encoder is input to the timing controls of the adhesive applicator. In this manner, a change of speed of the product passing by the adhesive applicator results in a corresponding change in the discharge of the adhesive applicator so as to correctly position the lines or dots of adhesive regardless of changes of speed in the product conveyor system.
The importance of properly controlling the dispensing of the adhesive is more clearly understood when viewed in consideration of the manufacture of products such as baby diapers. In such manufacture, a continuous web of non-woven material is moved past one or more applicator heads at high speeds. Control signals having a relatively constant time delay cause the applicator head to open and the adhesive, being under pressure, is applied to the fast moving web of material. Providing that the pressure remains constant and that there are no other variations in the system and providing the web (product) speed of travel remains constant, the lines or dots of adhesive are accurately and uniformly placed where required and in an amount that is required. However, in the event of a change in web or product speed, which can and often does occur in production lines, the adhesive will be misplaced on the product unless provision is made to correct for the speed change.
Speed variations typically occur during conveyor line startup and shutdown, during planned conveyor line speed adjustments and during unplanned speed adjustments, such as for malfunctions, product component co-ordination, equipment failure, shift changes, etc. An uncompensated faster or slower speed results in misplaced adhesive. This occurs because the location of the applied adhesive is dependent upon the time delay between the control signal and the opening of the applicator head which remains reasonably constant and upon the speed of the product which does not remain constant. Thus, if the product moves faster, the adhesive applied will be placed behind the correct location in proportion to the amount that the product speed has increased. Similarly, if the product speed decreases, the adhesive will be placed forward of the correct location. Further, if the length of the laid down line of adhesive is controlled as a function of time, not only will the starting point of the line of adhesive be improper but the length (and consequently the amount) of adhesive will similarly be improper.
The use of the previously mentioned shaft speed encoder is one prior art attempt to solve the above-stated problem. The shaft speed encoder being synchronized with the conveyor or product speed senses a change in product speed and, through appropriate electronic circuitry, adjusts the timing of the activation of the adhesive applicator so as to correctly reposition the location of the adhesive on the moving product and to lay down a correct length of adhesive.
Another prior art problem concerns the amount of adhesive applied relative to variations in the speed of the product along the conveyor line. If the conveyor line speed changes, and the location of the length of the line of adhesive is correctly adjusted by suitable control means, and if no other corrections are made, then a given length of adhesive will contain either more or less adhesive relative to the change in web speed. For example, a change in product speed of twenty percent would result in a twenty percent change in the amount of adhesive placed at a particular location. In practice, too much adhesive is costly; while too little adhesive can cause a product failure. For every application there is a preferred line or dot of adhesive placed in a correct starting and ending position and in an amount of adhesive which is optimal for that application. A usual prior art method of attempting to insure the dispensing of a correct amount of adhesive relative to the speed of the product line involves changing the pressure of the adhesive within the adhesive dispensing system in accordance with controls again influenced by the shaft speed encoder.
For a given timed opening of an applicator head, an increase in pressure will result in more adhesive being dispensed. Alternatively, a decrease in pressure results in less adhesive being dispensed. For example, in a system utilizing a DC motor to drive a gear pump, speeding up or decreasing the speed of the DC motor correspondingly changes the speed of the gear pump which increases the volume pumped and hence the pressure output thereby. By using the shaft encoder to control both the timed sequence of operation or opening of an adhesive applicator and the speed of the gear pump, both prior art problems are solved to some degree. Unfortunately, this prior art solution is not a complete solution. The relationship between pressure and pump speed is not a linear relationship. Also, the relationship between pressure and fluid flow rate is not linear. Hence, the use of pump speed in response to conveyor line speed only to change the pressure to compensate for variations in conveyor line speeds will not completely eliminate variations in the amount of adhesive deposited for a given length of line of adhesive.
The above-stated prior art problem exists not only in adhesive dispensing systems but any system which involves the dispensing of a predetermined quantity of liquid while a conveyor line is in motion. For example, the dispensing of eye shadow or eye liner into a container moving along a conveyor line.
It is, therefore, an object of the present invention to provide a method and control apparatus for use with an adhesive dispensing system for applying a given length or dot of adhesive at a predetermined and preferred position on a product driven past an adhesive applicator regardless of any variation in conveyor line speed.
Another object of the present invention is to provide a method and apparatus for use with an adhesive dispensing system for applying a predetermined and constant amount of adhesive per length of line of adhesive or per size of dot of adhesive application regardless of any variation in conveyor line speed.
It is still another object of the present invention to provide a method and apparatus for use with an adhesive dispensing system which will apply a predetermined length and a predetermined quantity of adhesive at a predetermined location on a product being driven past an adhesive applicator head regardless of any variation in conveyor line speed.
An even further object of the present invention is to provide a method and apparatus for use with a liquid dispensing system which will apply a desired and preferred quantity of a liquid to a container moving along a conveyor line while said container is in motion and regardless of any variation in conveyor line speed.